Evaporation characteristics of heptane droplets with the addition of aluminum nanoparticles at elevated temperatures

The evaporation characteristics of n-heptane droplets with varying concentrations of aluminum (Al) nanoparticles (NPs) hanging at a silicon carbide fiber were studied experimentally at different environmental temperatures (100–600°C) under normal gravity. The evaporation of pure and stabilized heptane droplets has been also examined for comparison. The characteristics of the shell formation due to evaporation of the NPs suspensions and its effects on evaporation rate were also investigated. The results show that the evaporation of suspended heptane droplets containing Al NPs follows the classical d2-law at all temperatures. The phenomenon of bubble formation in stabilized heptane droplets is reduced with the addition of Al NPs. For all Al NPs suspensions; regardless of their concentrations, the evaporation rate obtained was lower than pure heptane droplets from 100 to 300°C, but it monotonically increased and became higher than the evaporation rate of pure heptane droplets above 400°C. However for 2.5% (by weight) Al NPs suspension, the increasing trend in evaporation rate is exponential above 400°C. At relatively low temperatures the formation of large agglomerates results in a compact shell development which suppresses the evaporation. On the other hand, at high temperatures a highly porous shell was formed by small agglomerates so that Al NPs lead to evaporation enhancement. Maximum reduction of ∼15.5% in the evaporation rate at 200°C with 5% Al NPs and maximum increase of ∼50% in the evaporation rate at 600°C with 2.5% Al NPs suspension was observed.